Coaxial filter having harmonic reflective and absorptive means

ABSTRACT

A coaxial filter is disclosed. The filter includes a coaxial line having resonant reflector means carried from the outer conductor for reflecting harmonic wave energy toward the source while permitting fundamental wave energy to pass through the coaxial line to a load. A harmonic resonant means is disposed between the harmonic reflectors and the source for picking up and absorbing the harmonic energy in a load disposed externally of the outer conductor.

United States Patent [72] Inventor Richard Z. Gerlack Cupertino, Calif.[21] Appl. No. 849,715 [22] Filed Aug. 13, I969 [45] Patented Aug. 17,1971 [73] Assignee Varian Associates Palo Alto, Calif.

[54] COAXIAL FILTER HAVING HARMONIC REFLECTIVE AND ABSORPTIVE MEANS 6Claims, 9 Drawing Figs.

52 us. 0 333173 c, 333/76, 333/81 A [51] Int. Cl ..H03h 13/00, H0 1p1/22 [50] Field of Search 333/73, 73 W, 76, 81

so References Cited UNITED STATES PATENTS 2,588,226 3/1952 Fox 333/73FREQUENCY 2,785,381 3/1957 Brown 333/73 2,853,678 9/1958 Tomiyasu 333/73X 2,853,682 9/1958 Epstein 333/73 X 2,869,085 1/1959 Pritchard et al.333/73 X 2,961,619 11/1960 Breese etal. 333/73 X 3,078,423 2/1963 Lewis333/73 X 3,353,123 11/1967 Met 333/73 3,437,959 4/1969 Webb 333/73Primary ExaminerI-1erman Karl Saalbach Assistant Examiner-William H.Punter AttorneysStanley Z. Cole and Gerald L. Moore ABSTRACT: A coaxialfilter is disclosed. The filter includes a coaxial line having resonantreflector means carried from the outer conductor for reflecting harmonicwave energy toward the source while permitting fundamental wave energyto pass through the coaxial line to a load. A harmonic resonant means isdisposed between the harmonic reflectors and the source for picking upand absorbing the harmonic energy in a load disposed externally of theouter conductor.

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INVENTOR RICHARD Z. GERLACK %vubk If Am ATTORNEY COAXIAL FILTER HAVINGHARMONIC REFLECTIVE AND ABSORPTIVE MEANS DESCRIPTION OF THE-PRIOR ARTHeretofore, coaxial line harmonic reflective filters have been proposed.In one such filter, resonant second harmonic reflective antennas werecarried from the inner conductor of a coaxial line for reflecting secondharmonic wave energy toward the source while permitting fundamental waveenergy to pass through the coaxial filter to the load. An antennaresonant at the reflected second harmonic wave energy was disposed onthe center conductor and coupled to a load contained within the centerconductor for absorbing the reflected second harmonic wave energy. Theproblem with this prior art filter was that it had limited powerhandling capability due to the fact that the second harmonic wave energywas absorbed within the center conductor and cooling of the centerconductor became extremely difficult.

SUMMARY OF THE PRESENT INVENTION I The principal object of the presentinvention is the provision of an improved coaxial filter having harmonicreflective and absorptive means.

One feature of the present invention is the provision in a coaxialfilter having harmonic reflective means of resonant pickup meansdisposed between the harmonic reflectors and the source, such pickupmeans being disposed on the outer conductor for coupling reflectedharmonic wave energy out of the coaxial line to a load disposedexternally thereof, whereby the power handling capability of the coaxialfilter is enhanced.

Another feature of the present invention is the same as the precedingfeature wherein the resonant pickup means includes antenna meansdisposed inside the outer conductor, such antenna means having a firstpickup antenna tuned for resonance at the second harmonic and a secondpickup antenna tuned for resonance at the third harmonic.

Another feature of the present invention is the same as the immediatelypreceding feature wherein the first and second pickup antennas areaxially and circumferentially displaced from each other with respect tothe longitudinal axis of the coaxial line.

Another feature of the present invention is the same as the firstfeature wherein the resonant pickup means includes a resonant pickupantenna affixed at one end to the outer conductor and curving to a freeend in the circumferential direction in between the inner and outerconductors of the coaxial line, whereby the axial space of the coaxialline occupied by the antenna is reduced to a minimum.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specifications taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram,partly in block diagram form and partly broken away, depicting a coaxialfilter incorporating features of the present invention,

FIG. 2 is a sectional view of the structure of FIG. 1 taken along line2-2 in the direction of the arrows,

FIG. 3 is a sectional view of the structure of FIG. 1 taken along line3-3 in the direction of the arrows, and depicting an alternative pickupantenna embodiment of the present inventron,

FIG. 4 is a schematic line diagram, similar to that of FIG. 1, anddepicting an alternative embodiment of the present invention,

FIG. 5 is a plot of insertion loss in db. versus frequency depicting theperformance characteristics for the filter of FIG. 1,

FIG. 6 is a plot of insertion loss in db. versus frequency depicting theperformance characteristics of the filter of FIG. 4,

FIG. 7 is a detail view of a portion of the structure of FIG. 1depicting an alternative reflective antenna embodiment,

FIG. 8 is a detail view of a portion of the structure of FIG. 1

delineated by line 8-8 and depicting an alternative antenna embodiment,and

FIG. 9 is a detail view of a portion of the structure of FIG. 1delineated by line 9-9 and depicting an alternative reflective antennaembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2there is shown a coaxial filter l incorporating features of the presentinvention. The coaxial filter l is connected at one end to a source ofhigh frequency power 2, such as a UHF klystron, delivering power at afundamental output frequency f as of 500 megahertz, such fundamentalenergy being accompanied by the harmonics of the fundamental which it isdesired to absorb or to at least prevent from reaching a load 3connected to the output terminal of the filter. In a typical example,the fundamental energy is pulsed having a peak power of 2 megawatts with50 kilowatts average power and containing 100 watts second harmonic and3050 watts third harmonic power.

The harmonic filter 1 includes a section of coaxial line 4 having anouter conductor 5, as of 3% inches OD, and an inner conductor 6, as of1.3 to 1.5 inches OD. The coaxial line 4 can have any convenient lengthas of 24 inches. A plurality of reflective antennas 7 tuned for thesecond harmonic, are carried from the outer conductor 5 and are axiallyspaced along the length of the coaxial line 4. Successive ones of thereflective antennas 7 alternate from one side of the line to adiametrically opposed side of the line 4 in the axial direction. Theaxial spacing between successive second harmonic resonant reflectiveantennas 7 is approximately one-fourth of a wavelength at the secondharmonic such that the second harmonic wave energy reflected from theantennas adds in the reflected direction toward the high frequencysource 2. In a preferred embodiment, the reflective antennas 7 comprisequarter-wavelength sections of conductor affixed at one end to the outerconductor 5 and extending the axial direction between the inner andouter conductor and terminating at a free end.

A second set of wave reflective antennas 8, similar to the secondharmonic antennas 7, are tuned to be resonant at the third harmonic.Successive third harmonic antennas 8 alternate from one side to theopposite side of the outer conductor 5 in a circumferential positionrelative to the first set of second harmonic antennas 7. As in the caseof the second harmonic reflective antennas 7, the third harmonicreflective antennas 8 have an axial spacing between successive antennasof a quarter wavelength at the third harmonic frequency such that thewave energy reflected from successive reflective antennas 8 adds in thereflected direction taken toward the high frequency source 2.

A second harmonic resonant pickup antenna 9 is disposed in thelongitudinal plane of the array of second harmonic reflective antennas 7and is spaced from the reflective antennas 7 between the antennas 7 andsource 2 to pick up the reflected energy at the second harmonicfrequency. A conductor 11 is affixed to the pickup antenna 9 for tappingthe wave energy therefrom to a load 12, such as a conventional 50-ohmcoaxial load, for absorbing the reflected second harmonic wave energypicked up by antenna 9.

A similar third harmonic pickup antenna 13 is axially spaced from thereflective antennas 8 between the reflective antennas 8 and the highfrequency source 2 to pick up the reflected third harmonic wave energy.A conductive tap l4 taps off the antenna 13 for coupling third harmonicwave energy picked up by antenna 13 to a coaxial load 15, for absorbingthe reflected third harmonic wave energy. Loads at 12 and 14 may beconventional coaxial terminations or may comprise waterloads forhandling higher power. In a typical example of the filter of FIG. 1, thesecond harmonic load 12 absorbs watts of second harmonic power and thethird harmonic load 15 absorbs 30 to 50 watts of third harmonic powerwhen the fundamental power is 50 kilowatts average at 500 megahertz.

and curving in the circumferential direction between the inner and outerconductors 6 and 5, respectively, to a terminating free end. Theadvantage of the pickup antenna 16 is that it requires substantiallyless length of the coaxial line 4 than that required for the axiallydirected pickup antennas 9 and l3.

Referring now to FIG. 4, there is shown an alternative embodiment of acoaxial filter l8 incorporating features of the present invention. Thestructure of FIG. 4 is substantially identical to that of FIG. 1 withthe exception that the harmonic reflective structure 19 comprises aseries of conductive discs 21 of various axial thicknesses and axiallyspacings carried upon a section of the inner conductor 6 of reducedthickness to provide a reflective harmonic structure that reflects thesecond through the sixth harmonic. Such a coaxial reflective filterstructure is described in a text titled Microwave Filters, ImpedanceMatching Networks and Coupling Structures by Mattahei, G. L., L. Youngand E. M. T. Jones, published by McGraw-Hill Company, Inc. of New York,N.Y., in l964.

i As in the embodiment of FIG. 1, pickup antennas 9 and 13 for thesecond and third harmonic, respectively, are spaced by the appropriatedistance from the harmonic reflective structure 19 to pick up and absorbthe second and third harmonic wave energy reflected from the reflector19. If desired, additional harmonic pickup antennas for absorbing higherharmonic wave energy may be spaced at various circumferential locationsabout the center conductor 6 for picking up and absorbing the additionalharmonic wave energy.

Referring now to FIG. 5, there is shown a plot of insertion loss in db.versus frequency for the filter 1 of FIG. 1. From the plot of FIG. 5 itis seen that the fundamental wave energy indicated at 22 passessubstantially without attenuation to the load, whereas the second andthird harmonics at 2}], and 3]}, are heavily attenuated.

Referring now to FIG. 6, there is showna plot of insertion loss in db.versus frequency for the filter embodiment of FIG. 4. In thisembodiment, the fundamental wave energy 22 passes through the filterwithout attenuation, whereas the second through the sixth harmonics aresubstantially totally reflected and the second and third harmonics aresubstantially totally absorbed by the external loads 12 and 15,respectively.

Referring now to FIG. 7, there is' shown an alternative reflectiveantenna structure to that depicted in FIG. 1. In this embodiment, thereflective antenna is approximately half a wavelength long at theparticular harmonic and is connected to the outer conductor 5 via acentrally disposed stub 23.

Referring now to FIG. 8, there is shown an alternative wave reflectiveantenna defined by a section of coaxial line 24 dimensioned to beapproximately a half wavelength long at the respective harmonicfrequency and being connected by a short length of coaxial line 25 tothe outer conductor 5.

Referring now to FIG. 9, there is shown an alternative wave reflectivestructure 26 which comprises a quarter wavelength folded coaxial linesection 27 disposed surrounding the outer conductor 5 and coupled to thewave energy within the coaxial line via an annular coupling hole 28.

The resonators of FIGS. 7 through 9 may also be employed for replacingthe pickup resonators, in which case each of the respective resonatorswould be coupled to a suitable lossy material or load exposed externallyof the outer conductor 5. In the case of the embodiment of FIG. 9, thelossy material may be disposed inside the quarter wavelength section offolded coaxial line 27.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What I claim is: 1. In a coaxial filter, means forming a coaxial linehaving an inner conductor and outer conductor for transmission offundamental mode energy from a source to a load, means forming a wavereflective structure disposed in said coaxial line for reflectingharmonics of said fundamental mode energy toward the source, meanscoupled to said coaxial line between the source and said harmonicreflective means for picking up and absorbing reflected harmonic waveenergy, TI'IE IMPROVE- MENT WI-IEREIN, said harmonic pickup andabsorbing means include, resonant pickup means disposed on said outerconductor of said coaxial line, such pickup means being resonant atleast at a respective one of said harmonics, load means disposedexternally of said outer conductor, and means for coupling harmonic waveenergy from said resonant pickup means to said load means for absorbingreflected harmonic energy.

2. The apparatus of claim 1 wherein said resonant pickup means includesantenna means disposed inside said outer conductor and having a firstpickup antenna tuned for resonance at the second harmonic, and a secondpickup antenna tuned for resonance at the third harmonic.

3. The apparatus of claim 2 wherein said first and second pickup antennaare axially and circumferentially displaced from each other with respectto the longitudinal axis of said' coaxial line.

4. The apparatus of claim 3 wherein said harmonic reflective meansincludes a plurality of resonant reflective antennas extending into saidcoaxial line from said outer conductor, said antennas being axiallyspaced along said coaxial line and including a plurality of axiallyaligned reflective antennas resonant at the second harmonic and aplurality of aligned reflective antennas resonant at the third harmonic,said second harmonic reflective antennas being circumferentiallydisplaced relative to said third harmonic reflective antennas.

5. The apparatus of claim 1 wherein said resonant pickup means includesan antenna member affixed at one end to said outer conductor and curvingto a free end in the circumferential direction between said inner andouter conductors.

6. The apparatus of claim 1 wherein said pickup means includes aconductive antenna member affixed at one end to said outer conductor ofsaid coaxial line and extending to a free end in the axial direction ofsaid coaxial line in the space between said inner and outer conductorsof said coaxial line.

1. In a coaxial filter, means forming a coaxial line having an innerconductor and outer conductor for transmission of fundamental modeenergy from a source to a load, means forming a wave reflectivestructure disposed in said coaxial line for reflecting harmonics of saidfundamental mode energy toward the source, means coupled to said coaxialline between the source and said harmonic reflective means for pickingup and absorbing reflected harmonic wave energy, THE IMPROVEMENTWHEREIN, said harmonic pickup and absorbing means include, resonantpickup means disposed on said outer conductor of said coaxial line, suchpickup means being resonant at least at a respective one of saidharmonics, load means disposed externally of said outer conductor, andmeans for coupling harmonic wave energy from said resonant pickup meansto said load means for absorbing reflected harmonic energy.
 2. Theapparatus of claim 1 wherein said resonant pickup means includes antennameans disposed inside said outer conductor and having a first pickupantenna tuned for resonance at the second harmonic, and a second pickupantenna tuned for resonance at the third harmonic.
 3. The apparatus ofclaim 2 wherein said first and second pickup antenna are axially andcircumferentially displaced from each other with respect to thelongitudinal axis of said coaxial line.
 4. The apparatus of claim 3wherein said harmonic reflective means includes a plurality of resonantreflective antennas extending into said coaxial line from said outerconductor, said antennas being axially spaced along said coaxial lineand including a plurality of axially aligned reflective antennasresonant at the second harmonic and a plurality of aligned reflectiveantennas resonant at the third harmonic, said second harmonic reflectiveantennas being circumferentially displaced relative to said thirdharmonic reflective antennas.
 5. The apparatus of claim 1 wherein saidresonant pickup means includes an antenna member affixed at one end tosaid outer conductor and curving to a free end in the circumferentialdirection between said inner and outer conductors.
 6. The apparatus ofclaim 1 wherein said pickup means includes a conductive antenna memberaffixed at one end to said outer conductor of said coaxial line andextending to a free end in the axial direction of said coaxial line inthe space between said inner and outer conductors of said coaxial line.